1. Field of the Invention
The invention relates to an electrochemical measuring sensor for determining nitrogen oxides in gas mixtures, particularly in exhaust gases of internal combustion engines.
2. Description of the Related Art
It is known that exhaust gases of internal combustion engines, for example diesel engines, contain nitrogen oxides in addition to unburnt fuel components and oxygen. The composition of the exhaust gas is primarily determined by the setting of a fuel-air mixture with which the internal combustion engine is operated. If, for example, fuel is present in a stoichiometric surplus, significant quantities of unburnt or only partially-burnt fuel is present in the exhaust gas, whereas in a stoichiometric surplus of oxygen of the air in the fuel-air mixture, a correspondingly higher concentration of oxygen is noted in the exhaust gas. For setting an optimal fuel-air mixture, it is known to determine the composition of the exhaust gas with a limiting current sensor (.lambda. sensor). The limiting current sensor includes a solid electrolyte which is disposed between two electrodes, with one electrode being exposed directly to the exhaust gas and another electrode being exposed to the exhaust gas by way of a diffusion barrier. If a constant voltage is applied to the electrodes, a limiting current is established at the two electrodes due to a difference in oxygen concentrations; this current is measured with a measuring device and evaluated, for example to set the fuel-air mixture with which the internal combustion engine is operated.
A limiting current sensor of this type is known from, for example, DE-PS 37 28 618. In this case, an electrode configured as a pump electrode is disposed in a diffusion channel which is connected on one side to the gas mixture to be measured. Disposed inside the diffusion channel is a diffusion barrier which is designed such that an oxygen partial pressure corresponding to the applied voltage is established at the electrode connected to the exhaust gas by way of the diffusion barrier.
Furthermore, measuring sensors are known in which the one electrode is exposed to the gas mixture to be measured and the other electrode is exposed to a reference gas. Because a difference in oxygen concentrations is established at the electrodes, a voltage is applied to the electrodes which provides a measure for the oxygen concentration in the gas mixture to be measured.
A disadvantage of the known measuring sensors is that they can only be used to determine the oxygen content of the gas mixture to be measured, while other components of the gas mixture are not taken into consideration.